Isotropic neutron spectrometer

ABSTRACT

An isotropic neutron spectrometer having a plurality of radial polyethylene rods positioned in bore holes angularly spaced around a polyethylene sphere with thermoluminescent dosimeter pairs positioned at spaced intervals along the rods. A TLD pair is also located in the center of the sphere.

United States Patent Piltingsrud Feb. 12, 1974 ISOTROPIC NEUTRONSPECTROMETER [75] Inventor: Harley V. Piltingsrud, Idaho Falls,

Idaho [73] Assignee: The United States of America as represented by theSecretary of the v United States Air Force,

Washington, DC.

[22] Filed: Nov. 3, 1972 [21] Appl. No.: 303,438

[52] US. Cl. 250/390, 250/484 v [51] Int. Cl G0lt 3/00, GOlt 1/11 [58]Field of Search.... 250/337, 390, 391, 392, 484

[56] References Cited UNITED STATES PATENTS 3,290,500 12/1966 Bokhari eta]. 250/39l 3,420,999 1/1969 Distenfeld t. 250/391 PrimaryExaminer-Archie R. Borchelt ABSTRACT An isotropic neutron spectrometerhaving a plurality of radial polyethylene rods positioned in bore holesangularly spaced around a polyethylene sphere with thermoluminescentdosimeter pairs positioned at spaced intervals along the rods. A TLDpair is also located in the center of the sphere.

4 Claims, 8 Drawing Figures PATENIE FEB x 21914 SHEEI 2 BF 2 m tf'ISOTROPIC NEUTRON SPECTROMETER BACKGROUND OF THE INVENTION determiningaverage neutron energy and fluence in a radiation field.

Six TLDs of LiF made of Li and two TLDs of LiF made of Li are placed atintervals along a single polyethylene rod which is inserted in a bore inthe 18 inch sphere. A like packet is taped at the end of the rod. Theuse of such apparatus, however, requires the use of rotating machinerywhich makes the apparatus not practicable for certain uses, such as athigh altitudes aboard an aircraft, due to the power and spacerequirements of the rotating machinery. Also, since this apparatus isrotated around a single axis so that the rod moves in only one plane,such a system will not provide an isotropic response.

Furthermore, these systems do not provide for the determination of thedirectional properties of the incident radiation.

BRIEF SUMMARY OF THE INVENTION According to this invention, TLD pairs,of LiF made of Li and LiF made of Li, are positioned at one inchintervals along a plurality of polyethylene rods. The rods are locatedin radial bore holes at angularly spaced positions in a polyethylenesphere. A TLD pair is positioned at the center of the sphere. A TLD pairis also positioned at the outer end of each of the rods.

IN THE DRAWING FIG. 1 is a front elevation view ofa spherical isotropicneutron spectrometer according to the invention.

FIG. 2 is a sectional view of the device of FIG. I along the lines 2--2with the rods shown in full.

FIG. 3 is a sectional view'of the device of FIG. I along the line 33with the rods shown in full.

FIG. 4 is a sectional view of the device of FIG. 1 along the line 4-4with the rods shown in full.

FIG. 5 is an enlarged side view of one of the rods of FIG. 1.

FIG. 6 is an enlarged side view of the rod of FIG. 1 which is positionedalong a diameter of the sphere.

FIG. 7 is a partially cut away enlarged sectional view of the device ofFIG. 6 along the line 7-7.

FIG. 8 is a partially cut away sectional view of a modification of theinvention, corresponding to the showing in FIG. 2. I

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1 of thedrawing, a large polyethylene sphere is shown at 10. The sphere has onebore hole 12 passing through the sphere as shown in FIG. 3 and four boreholes 14a and eight bore holes 14b extending to a position adjacent thecenter of the sphere, as shown in FIGS. 2, 3 and 4. The bore holes 14aare positioned in a plane perpendicular to bore hole 12 and shown inFIG. 2. Bore holes 14b are located at, angles of 45 with respect to theplanes in FIGS. 2 and 3.

With the bore holes located as described, an approximately uniformdistribution of the TLD pairs in the sphere is provided. However, theideal distribution would be with the TLD pairs located in bore holes atequal angular spaced positions in the sphere. Where needed these may beprovided.

The rod 16 positioned in bore hole 12 has cylindrical recesses 20 spacedat one inch intervals along the rod on two opposite sides, as shown inFIGS. 6 and 7. Polyethylene cover members 22 close the recesses to holdTLD chips 24 and 25. The TLDs 24 of LiF made of Li are positioned in therecesses on one side of the rod 16 and the TLDs 25 of LiF made of Li arepositioned in the recesses on the opposite side of the rod 16. A TLDpair is located in each of the recesses 27 at the opposite ends of rod16 and are held by cover members 29. While the TLD pairs were located atequal intervals along the rod, other spacing may be used in someapplications.

Rods 18 positioned in bore holes 14a and 15b are as shown shown in FIG.5. Rods 18 also have recesses 20 and 27 I with covers 22 and 29 forholding TLDs in the same manner as rod 16.

In the operation of the apparatus, rods 16 and 18 with the TLDs in placeare positioned in the bore holes in sphere 10. The sphere is thenpositioned in the area where it is desiredto determine the neutronfields, such as aboard an aircraft for flight at a high altitude oraround a high-energy particle accelerator. After the sphere has beenexposed to such radiation, the radiation field is evaluated by readingeach TLD in an analyzer such as described in the Distenfeld patent. TheTLDs made of Li measures gammas and charged particles only and the Limeasures gammas, charged particles and thermal neutrons. The differencebetween the Li reading and the Li reading in the analyzer is due to thethermal neutrons. By averaging the reading at equal distance from thecenter of the sphere, an indication similar to that derived in therotating apparatus is achieved except that a more nearly isotropicresponse is obtained. i

By observing the readings as a "function of response direction, adirectional response characteristic may be obtained. In devicesconstructed, two sphere sizes have been used, some with 8 inchesdiameter and some with 18 inches diameter. While the smaller spheres arecheaper to build and easier to handle and are useful in lower energyfields, the larger spheres are needed for evaluating radiation fieldswhere very high energy neutrons are present.

In one device constructed, all the rods were the same and located inradial bore holes 14a and 14b around a small sphere 31 located in thecenter of the large sphere with the center TLD pair 32 being located inthe center of the small sphere.

While the TLD chips have been described as being located in recesses inthe rods, other holders may be provided, for example, in one devicebuilt cross bores were provided in the rod with small round rod shapedTLD containers being located in the cross bores.

The spheres were fabricated of laminated l 2% inches thick section ofpolyethylene sheet. This greatly reduced the fabrication and materialcosts.

There is thus provided an isotropic neutron spectrometer that is acompletely passive device which may be used in the measurement of fluxand energy spectrum of neutrons in mixed radiation fields.

I claim:

1. An isotropic neutron spectrometer comprising: a polyethylene spherehaving a predetermined diameter; said sphere having a plurality ofradially disposed bore holes angularly spaced around said sphere; apolyethylene rod positioned in each of the bore holes; means for holdingTLD pairs of LiF made with Li and LiF made with Li at spaced intervalsalong said rods with the TLD pairs on all the rods being spaced atcorresponding equal distances from the center of the sphere; means forholding a like TLD pair at the outer end of each rod; means for holdinga like TLD pair at the center of said sphere.

2. The device as recited in claim 1, wherein one of said rods passesthrough the sphere along a diameter with the TLD pair at the center ofthe sphere being held by said rod. 3. The device as recited in claim 1,wherein a small polyethylene sphere is positioned at the center of saidpolyethylene sphere with all of said rods abutting said small sphere;the TLD pair in the center of the said sphere being held by said smallsphere.

4. The device as recited in claim 1, wherein the TLD spaced along therod are held in a covered recess in said rods with the TLDs with Libeing located on one side of the rods and the TLDs with Li being locatedon the opposite side of the rods.

2. The device as recited in claim 1, wherein one of said rods passesthrough the sphere along a diameter with the TLD pair at the center ofthe sphere being held by said rod.
 3. The device as recited in claim 1,wherein a small polyethylene sphere is positioned at the center of saidpolyethylene sphere with all of said rods abutting said small sphere;the TLD pair in the center of the said sphere being held by said smallsphere.
 4. The device as recited in claim 1, wherein the TLD spacedalong the rod are held in a covered recess in said rods with the TLDswith Li6 being located on one side of the rods and the TLDs with Li7being located on the opposite side of the rods.